In many environments, various materials are stored and/or processed in tanks prior to or during their disposition. These materials include foods, beverages, pharmaceuticals and fuels. One particular and commonly-known use of such tanks involves the storage of fuel for various modes of transportation. These fuel tanks can range from a gas tank on a car to voluminous fuel tanks such as those located on Naval ships. In particular, aircraft carriers house multiple fuel tanks. These are used to store jet fuel for the aircraft carried on the ship.
Regarding these fuel tanks on Naval ships, the ability to reliably determine the amount and purity of the contents stored within a tank at any given time can be critical. Military readiness is often dependent on an adequate fuel supply. Additionally, accurate measurements of fuel usage directly translate into cost effectiveness in procuring a fuel supply for ships. As such, there are several benefits to accurately and reliably gauging the levels of fuel in these tanks. Thus, the Navy measures to determine fuel levels. Additionally, the operation of high-performance jets is dependent on a low level of impurities in the fuel. Contaminates in the fuel can cause damage to many components of jets and can disrupt the performance of the jets in a manner that is hazardous to the occupants. Thus, the Navy frequently measures the purity of fuel within storage tanks.
Methods for the determination of liquid levels may include visual examination or the use of various apparatus that gauge the level of the fuel. Current electronic measurement methods include differential pressure and magnetic float transmitter technologies, both of which rely on sensor contact with the process medium. However, several factors, both in the structure of the tanks and in the methods of measurement used, increase the difficulty in obtaining an accurate and reliable level reading. In the particular situation of jet fuel tanks of Navy ships, any visual reading is obstructed by the location of the tanks within the bowels of the ship and by the voluminous size of the tanks. Additionally, visual inspection of content levels lacks accuracy and can be time consuming. In order to avoid the problems attendant visual examination, various apparatus may be used to measure content levels in fuel storage tanks.
For example, differential pressure transmitters measure static pressure head of the liquid in the tank and pressure (if any) of gases above the liquid. The arithmetic difference between these values is used to determine the liquid level. This is an inherently unreliable and inaccurate method because it depends on sensor location, ship motion and is an "inferred" level based on two other measurements.
Another example includes magnetic floats which are mechanical in nature and involve a ball or buoy floating on the surface of the liquid, typically in a guide or sleeve mounted to the tank wall. These systems require cumbersome incremental metered filling of the tank to establish electrical resistance values that correspond to tank level. They are also prone to clogging failures which require expensive invasive repair work.
Other problems with accuracy arise from a method commonly used to measure fuel level on Naval ships. By this method, the top of the sounding tube is opened. A plumb bob is then dropped through the interior of the sounding tube to the bottom of the sounding tube. Once the plumb bob is retracted from the bottom of the sounding tube using the plumb line, the liquid level may be read from the moisture level created by the fuel on the plumb line. However, may problems arise from this method of liquid level measurement. First, jet fuel is clear and evaporates very rapidly, thereby enhancing the difficulty of accurately reading the plumb line to determine the associated moisture level. Second, the plumb bob may break off the plumb line during use. Due to the difficulty involved, as a practical matter detached plumb bobs are not retrieved from sounding tubes. As a result, subsequent plumb bobs used for measurement may be impeded from falling to the bottom of the sounding tube, resulting in reduced measurement range.
In order to overcome the drawbacks of current methods of liquid level measurement used by the Navy, Applicants, in above-referenced U.S. patent application Ser. No. 09/491,555, disclose the use of a level sensing gauge adapted to be operatively connected to a sounding tube for quickly and reliably determining fuel levels in storage tanks. The level sensing apparatus includes a latch door which seals off an orifice through which a plumb bob may be inserted to be employed as a redundant method of measurement.
While the use of a level sensing gauge on a sounding tube has proven adequate for measuring the liquid level of contents of storage tanks, one drawback of the invention is that it does not allow for samples of tank contents to be collected in order to determine their purity.
The purity of fuel stored on Naval ships is determined by a method commonly referred to as "thief sampling". In this process, an elongate hollow tube, or "thief sampler", is dropped into the contents of a tank. Fuel from the tank fills the hollow recess of the thief sampler. The thief sampler is then retracted and the purity of the collected sample is measured by methods well known in the art.
One drawback of the use of a level sensing apparatus on a sounding tube is that the apparatus closes off the end of the sounding tube, thereby preventing a thief sampler from being inserted to collect samples from the tank. As a result, in order to obtain a sample of fuel, the level sensing apparatus must first be removed from the sounding tube. This is a time consuming and laborious process. After the level sensing apparatus has been removed from the sounding tube, it must be set aside while the sample is being collected. This requires placing this highly sensitive piece of equipment on the deck of the ship. In this location, the level sensing gauge is vulnerable to damage. Finally, after a sample has been taken, the level sensing gauge must be reattached to the sounding tube. Not only is this time consuming, but the process may additionally require a recalibration of the level sensing gauge.
Another drawback to the use of a level sensing gauge on a sounding tube is that, due to its size and elongate shape, the thief sampler is not amenable to insertion through an orifice in the housing of the level sensing gauge. In order to take advantage of an orifice of sufficient size disposed in the housing of the level sensor, the length of the housing would need to be extended. This creates additional problems. For example, the level sensing apparatus is a bulky piece of equipment and is often located in areas which have little head space clearance. In such cramped locations, extending the length of the level sensing apparatus is not a feasible option.
Thus, it would be desirable for a level sensing apparatus to be amenable for attachment to a sounding tube in order to accurately and reliably measure the level of contents in storage tanks. It would also be desirable for the level sensing gauge to be easily removable for thief sampling without requiring complete, physical detachment of the gauge from the sounding tube. Further, it would be desirable for the apparatus to allow for operation in areas with low head space clearance.